Antireflection structure and optical device including the same

ABSTRACT

A diffusing plate is formed so as to have a surface having a larger surface roughness than a predetermined wavelength and having an aperiodic roughness shape. A plurality of fine concave/convex portions are formed on the surface so as to be regularly arranged within a cycle equal to and smaller than a predetermined wavelength.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an antireflection structure and anoptical device including the antireflection structure.

2. Description of Prior Art

In recent years, various kinds of optical devices in whichantireflection processing for suppressing reflection of light isperformed to a surface have been proposed. As antireflection processing,for example, processing for formation of an antireflection filmincluding a film having a relatively low refractive index (which will beherein referred to as a “low refractive index film”), a multilayer filmin which a low refractive index film and a film having a relatively highrefractive index (which will be herein referred to as a “high refractiveindex film”) are alternately stacked, or like film has been proposed(for example, see Japanese Laid-Open Publication No. 2001-127852 and thelike).

However, for formation of an antireflection film including a lowrefractive index film or a multilayer film, complex processing such asvapor deposition, sputtering and the like need to be performed. Thus,although productivity is low, production costs become high. Moreover, anantireflection film including a low refractive index film or amultilayer film has high dependency on wavelength and incident angle.

In view of the above-described problems, as antireflection processingrelatively less dependent on incident angle and wavelength, for example,processing in which fine concave/convex portions are regularly formed ona surface of an optical device with a pitch equal to or smaller than awavelength of incident light has been proposed (for example, Daniel H.Raguin and G. Michael Morris, “Analysis of antireflection-structuredsurfaces with continuous one-dimensional surface profiles”, AppliedOptics, vol. 32, No. 14, pp. 2582-2598, 1993, and the like). Byperforming this processing, abrupt change in refractive index in adevice interface can be suppressed, so that a refractive index isgradually changed at the device surface. Accordingly, reflection at asurface of an optical device is reduced and a high impingement rate forincident light into the optical device can be achieved.

In National Publication of Translated Version No. 2001-517319, atechnique in which fine concave/convex portions are formed on a roughsurface is disclosed.

SUMMARY OF THE INVENTION

However, even when fine concave/convex portions are formed on a roughsurface, there are cases where the generation of unnecessary light suchas reflection light and the like can not be sufficiently suppressed.

The present invention has been devised in view of the above-describedpoints and it is therefore an object of the present invention to providean antireflection structure in which the generation of unnecessary lightsuch as reflection light and the like is sufficiently suppressed.

As a result of keen studies, the present inventors found that when fineconcave/convex portions are formed on a rough surface, there are caseswhere diffracted light is generated and have reached the presentinvention.

Specifically, an antireflection structure according to the presentinvention is directed to an antireflection structure for suppressingreflection of light having a wavelength equal to or larger than apredetermined wavelength and is characterized in that the antireflectionstructure includes a rough surface having a larger surface roughnessthan the predetermined wavelength and having an aperiodic roughnessshape, and a plurality of fine concave/convex portions are formed on therough surface so as to be regularly arranged within a smaller cycle thanthe predetermined wavelength. Herein, a “predetermined wavelength” meansto be a wavelength of light of which reflection is desired to besuppressed, or a wavelength of light of which reflection should besuppressed.

An optical device according to the present invention is characterized byincluding the antireflection structure of the present invention.

Accordingly, an antireflection structure in which the generation ofunnecessary light such as reflection light and the like can besufficiently suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a diffusing plate 1.

FIG. 2 is a cross-sectional view of part of the diffusing plate 1.

FIG. 3 is a graph showing the correlation between incident angle andreflection coefficient.

FIG. 4 is a graph showing reflection light intensity of incident lightat an incident angle of 45 degrees.

FIG. 5 is an illustration of a spectrum obtained by Fourier-transforminga height distribution in the normal direction of a reference plane of ashape of the surface 10.

FIG. 6 is a cross-sectional view illustrating the case where θ is largerthan 90 degrees.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereafter, embodiments of the present invention will be described withreference to the accompanying drawings. Herein, using a diffusing plateimplemented in accordance with the present invention as an example, anembodiment of an antireflection structure according to the presentinvention will be described. However, the antireflection structureaccording to the present invention is not limited to the followingembodiments but may be applied to, for example, some other opticaldevice such as a semiconductor laser device, a LED device, an electricbulb, a cold-cathode tube and the like, an image sensor such as acharge-coupled device (CCD), a CMOS and the like, an optical detectorsuch as a power meter, an energy meter, a reflection coefficientmeasuring device and the like, a microlens array, a photo disc and thelike.

FIG. 1 is a schematic view of a diffusing plate 1 according to thisembodiment.

FIG. 2 is a cross-sectional view of part of the diffusing plate 1.

The diffusing plate 1 according to the present invention is a facematerial having an approximately rectangular shape when viewed from thetop. The diffusing plate 1 diffuses light and transmits diffused light(more specifically, at least diffuses and transmits light of whichreflection is suppressed by fine concave/convex portions 11 which willbe described later). For example, the diffusing plate 1 is placed on afront of a display and the like and suppresses reflection of light(glare caused by extraneous light) at a display surface. Note that amaterial of the diffusing plate 1 is not particularly limited but thediffusing plate 1 may be formed of resin or glass. Also, particles andthe like may be dispersedly mixed in the material.

In this embodiment, as shown in FIG. 2, a plurality of fineconcave/convex portions 11 are formed on a surface 10 of the diffusingplate 1 so as to be regularly arranged within a cycle equal to orsmaller than a wavelength of incident light 20 (the cycle of the fineconcave/convex portions 11 is preferably equal to or smaller than asmallest wavelength of incident light). (Hereafter, an antireflectionstructure in which the plurality of fine concave/convex portions 11 areformed will be occasionally referred to as “SWS”.) Thus, abrupt changein refractive index between the surface 10 of the diffusing plate 1 andan air layer can be suppressed, so that a refraction index is graduallychanged in a surface layer portion of the surface 10 including the fineconcave/convex portions 11. Thus, as shown in FIG. 3, with the fineconcave/convex portions 11 formed in the surface 10, reflection at thesurface 10 of the diffusing plate 1 can be effectively suppressed.

As long as the fine concave/convex portions 11 have the function ofmoderating change in refractive index at an interface between thesurface 10 and the air layer, a shape of each of the fine concave/convexportions 11 is not particularly limited. For example, each of the fineconcave/convex portions 11 may be an approximately conical concave orconvex (of which a top portion may be chamfered or R-chamfered), aprismoid concave or convex or a filiform concave or a filiform convex ofwhich a cross-sectional shape is triangular, trapezoidal, rectangular orthe like (of which edge portions may be R-chamfered).

In view of realizing high antireflection effect, a cycle (i.e., adistance between top points of adjacent ones of the fine concave/convexportions 11 when viewed from the top in the normal direction of areference plane of the surface 10 formed to be a rough surface) of thefine concave/convex portions 11 is preferably equal to or smaller than awavelength of incident light 20. Herein, a “reference plane” means to bea plane obtained by cutting off the fine concave/convex portions 11 anda roughness shape as high-frequency components. A height (strictlyspeaking, defined to be a distance from the reference plane of thesurface 10, which is formed to be a rough surface, in the normaldirection of the reference plane) of each of the fine concave/convexportions 11 is preferably equal to or larger than 0.4 times as large asa wavelength of the incident light 20, more preferably equal to orlarger than the wavelength, and even more preferably equal to or largerthan three times as large as the wavelength. Strictly speaking, as inthis embodiment, assume that the incident light 20 has a wavelengthwidth. The cycle of the fine concave/convex portions 11 is preferablyequal to or smaller than a smallest wavelength of incident light and theheight of each of the fine concave/convex portions 11 is preferablyequal to or larger than 0.4 times as large as the largest wavelength ofthe incident light 10 (more preferably the same as the largestwavelength and even more preferably equal to or larger than three timesas large as the largest wavelength.

The fine concave/convex portions 11 do not have to exhibitantireflection effect for all the incident light 20. For example, when awavelength of the incident light 20 is in a wide wavelength rangeincluding ultraviolet light, near-ultraviolet light, visible light,near-infrared light and infrared light but only reflection of lighthaving a wavelength of 400 nm or more and to 700 nm or less needs to besuppressed, the cycle of the fine concave/convex portions 11 ispreferably equal to or smaller than 400 nm. The height of each of thefine concave/convex portions 11 is preferably equal to or larger 0.4times as large as 700 nm, i.e., 280 nm or more.

The fine concave/convex portions 11 may be formed so that the height ofthe fine concave/convex portions 11 differs between different parts (forexample, each having a size of 1 mm squares) of the surface 10. However,in view of simplification of formation, the fine concave/convex portions11 are preferably formed so that respective heights of the fineconcave/convex portions 11 in the different parts are approximately thesame. When the fine concave/convex portions 11 include conical/pyramidalconcaves and conical/pyramid convexes, the fine concave/convex portions11 are preferably formed so that a central axis of each of cones orpyramids, connecting a center point of a base and a top point of each ofthe cones or the pyramids, is approximately in parallel to centralaxises of other cones or pyramids. In this case, fabrication of thediffusing plate 1 by injection molding is simple. For the same reason,when the fine concave/convex portions 11 include filiform concaves andfiliform convexes each having a triangular cross section, the pluralityof the fine concave/convex portions 11 are preferably formed so that acenter axis of each of filiform portions, connecting respective centerpoints of a top and a base of each of the filiform portions, isapproximately in parallel to center axises of other filiform portions ineach part (for example, having a size of 1 mm squares) of the surface10.

As has been described, the plurality of fine concave/convex portions 11are formed at the surface 10, so that reflection of light at the surface10 can be suppressed. However, when the surface 10 is a smooth surface,regular reflection at the surface 10 can not be sufficiently suppressed.

FIG. 4 is a graph showing reflection light intensity of incident lightat an incident angle of 45 degrees.

As shown in FIG. 4, when the fine concave/convex portions 11 are formedon a smooth surface, reflection light at an output angle of about 45degrees, i.e., regular reflection is observed. In this manner, when thesurface 10 in which the fine concave/convex portions 11 are formed is asmooth surface, regular reflection of the incident light 20 can not besufficiently suppressed. In contrast, as shown in FIG. 4, when the fineconcave/convex portions 11 are formed on a rough surface having a largersurface roughness than a wavelength of incident light, regularreflection is substantially not observed. In this embodiment, as shownin FIG. 2, the surface 10 is formed so as to be a rough surface having alarger surface roughness than a wavelength of incident light. Morespecifically, the surface 10 is formed so that a surface roughness interms of maximum height roughness Rz defined in ISO4287:1997 (JIS B0601:2001) is larger than a wavelength of the incident light 20. Thus, in thediffusing plate 1 of this embodiment, regular reflection at the surface10 can be sufficiently suppressed. Note that the effect of suppressingthe generation of regular reflection tends to be reduced when thesurface roughness of the surface 10 is too large. A preferable range ofthe surface roughness Rz of the surface 10 is 100 μm or less. Thesurface roughness Rz is more preferably 50 μm and even more preferably30 μm.

As shown in FIG. 3, when the fine concave/convex portions 11 (SWS) areformed on a smooth surface, a sufficient antireflection effect for lightat a relatively large incident angle can not be achieved. That is,reflection coefficient is dependent on incident angle. In contrast, asin this embodiment, the fine concave/convex portions 11 (SWS) are formedon the surface 10 which is a rough surface, so that, as shown in FIG. 3,a sufficient antireflection effect for light at a relatively largeincident angle can be also achieved. That is, according to thisembodiment, the dependency of reflection coefficient on incident anglecan be reduced and reflection of light at a relatively large incidentangle can be effectively suppressed.

As has been described above, with the surface 10 formed so as to be apredetermined rough surface, reflection of the incident light 20 at arelatively large incident angle as well as regular reflection can beeffectively suppressed. When a roughness shape of the surface 10 has apredetermined periodic structure, diffracted light might be generated.In this embodiment, the surface 10 is formed so as to have an aperiodicroughness shape. Thus, with the diffusing plate 1 of this embodiment,not only reflection can be effectively suppressed but also thegeneration of diffracted light can be effectively suppressed, so thatthe generation of unnecessary light such as reflection light, diffractedlight and the like can be effectively suppressed. Herein, a roughnessshape means to be a shape obtained by cutting off the fineconcave/convex portions 11 as high-frequency components from a shape ofthe surface 10 including the fine concave/convex portions 11 (hereafter,the shape of the surface 10 including the fine concave/convex portions11 will be referred to as merely a “shape of the inner circumferencesurface 10”).

In view of effectively suppressing the generation of diffracted light,as shown in FIG. 5, in a spectrum obtained by Fourier-transforming aheight distribution in the normal direction of the reference plane ofthe surface 10, a peak width (for example, a width of a peak at a halfof a height of the peak) W₂ of a peak 16 for the roughness shape of thesurface 10 is preferably larger than a peak width W₁ of a peak 15 forthe fine concave/convex portions 11.

In view of further reducing the generation of uneven diffracted light,the surface 10 is preferably formed so that a distribution width ofcycles standardized with a center cycle (which is most frequentlyincluded in the surface 10) of the surface 10 is equal to or larger than0.4 times as large as the center cycle. If the distribution width of thecycles standardized with the center cycle is smaller than 0.4 times aslarge as the center cycle, in the surface 10, part of a diffractionangle in which a second-order diffracted light exists and another partof a diffraction angle in which a third-order diffracted light existsare isolated from each other, so that unevenness in the generateddiffracted light might be generated. On the other hand, if thedistribution width of the cycles standardized with the center cycle isequal to or larger than 0.4 times as large as the center cycle, in thesurface 10, part of a diffraction angle in which the second-orderdiffracted light is generated and another part of a diffraction angle inwhich the third-order diffracted light is generated are partiallysuperimposed with each other, so that unevenness in diffracted light canbe reduced.

Particularly, the distribution width of the cycles standardized with thecenter cycle of the roughness shape of the surface 10 is preferablyequal to or larger than ⅔ times as large as the center cycle. With thisconfiguration, not only part of a diffraction angle in which thesecond-order diffracted light is generated and another part of adiffraction angle in which the third-order diffracted light is generatedare partially superimposed with each other but also part of adiffraction angle in which a first-order diffracted light is generatedand another part of a diffraction angle in which the second-orderdiffracted light is generated are partially superimposed with eachother, so that unevenness in diffracted light due to the absence of partof a diffraction angle in which neither the first-order diffracted lightnor the second-order diffracted light exist can be reduced.

In view of fabrication, as shown in FIG. 6, it is preferable that partin which an angle (θ) between a normal vector N₂ of a tangent plane 13of a surface roughness shape of the surface 10 and a normal vector N₁ ofa reference plane 12 of the surface 10 does not exist. In other words,it is preferable that the surface 10 is substantially formed of a planehaving a roughness shape of θ≦90 degrees. This is because, as shown inFIG. 6, when the part in which θ is larger than 90 degrees exists, it isdifficult to form the fine concave/convex portions 11 on part of thesurface 10 facing to a depression portion 17.

In this embodiment, the antireflection structure of the presentinvention has been described using the light transmitting diffusingplate 1 as an example. However, the antireflection structure of thepresent invention is not limited to a light transmitting structure butmay be, for example, a light absorbing structure, i.e., a so-calledblack body.

Moreover, in this embodiment, an example where the SWS is formeddirectly on the surface 10 of the diffusing plate 1 has been described.However, a seal in which the SWS is formed may be adhered or coheredonto a flat smooth surface to form the surface 10. In other words, thediffusing plate 1 does not have to be a single unit body, but may beformed of a plurality of components.

In this embodiment, an example where the SWS is formed throughout thesurface 10 has been described. However, the SWS does not have to beprovided throughout the surface 10, but may be formed only in necessarypart. In such case, as well as the part in which the SWS is provided,other part of the surface 10 may be a rough surface having the samesurface roughness as the surface roughness of the part in which the SWSis provided or may be a smooth surface having a smaller surfaceroughness than the surface roughness of the part in which the SWS isprovided. Furthermore, some other antireflection structure including amultilayer film of a film having a relatively low reflection coefficientand a film having a relatively large reflection coefficient may beformed in part in which the SWS is not formed. Moreover, even in thepart in which the SWS is formed, a height and a cycle (pitch) of the SWSmay be adjusted as necessary.

The present invention is not limited to the above-described embodimentand various modifications are possible without departing from the spiritand material features of the present invention. The above-describedembodiment is merely an example in all aspects and its interpretation isnot to be limited. The scope of the present invention is indicated bythe scope of claims and not limited by the specification. Furthermore,all changes and modifications belonging to the scope of equivalents ofthe claims fall within the scope of the present invention.

1. An antireflection structure for suppressing reflection of lighthaving a wavelength equal to or larger than a predetermined wavelength,the antireflection structure comprising: a rough surface having a largersurface roughness than the predetermined wavelength and having anaperiodic roughness shape, wherein a plurality of fine concave/convexportions are further formed on the rough surface so as to be regularlyarranged within a smaller cycle than the predetermined wavelength, andwherein the rough surface is formed so that a distribution width ofcycles standardized with a center cycle of the roughness shape is equalto or larger than 0.4 times as large as the center cycle.
 2. Theantireflection structure of claim 1, wherein the rough surface is formedso that a distribution width of cycles standardized with a center cycleof the roughness shape is equal to or larger than ⅔ times as large asthe center cycle.
 3. The antireflection structure of claim 1, wherein aheight distribution in a normal direction of a reference plane of therough surface of the light absorbing antireflection structure presents apeak for a roughness shape of the rough surface having a larger widththan a width of a peak for the fine concave/convex portions in thespectrum in a spatial frequency domain.
 4. The antireflection structureof claim 1, wherein the antireflection structure transmits the light ofwhich reflection is suppressed.
 5. The antireflection structure of claim1, wherein the fine concave/convex portions are approximatelyconical/pyramidal concaves and convexes, or filiform concaves andconvexes.
 6. The antireflection structure of claim 1, wherein a surfaceroughness of the rough surface in terms of a maximum height roughness Rzdefined by ISO4287:1997 is larger than the predetermined wavelength. 7.The antireflection structure of claim 1, wherein a surface roughness ofthe rough surface in terms of a maximum height roughness Rz defined byISO4287:1997 is smaller than 100 μm.
 8. The antireflection structure ofclaim 1, wherein a surface roughness of the rough surface in terms of amaximum height roughness Rz defined by ISO4287:1997 is smaller than 50μm.
 9. The antireflection structure of claim 1, wherein a surfaceroughness of the rough surface in terms of a maximum height roughness Rzdefined by ISO4287:1997 is smaller than 30 μm.
 10. The antireflectionstructure of claim 1, wherein respective heights of the plurality offine concave/convex portions from the rough surface in a normaldirection of a reference plane of the rough surface are approximatelythe same.
 11. The antireflection structure of claim 1, wherein the roughsurface is substantially formed of a plane in which an angle between anormal vector of a tangent plane of a roughness shape and a normalvector of a reference plane of the rough surface is 90 degrees or less.12. The antireflection structure of claim 1, wherein each of theplurality of fine concave/convex portions is a conical/pyramidal concaveor convex and respective center axes of the plurality of fineconcave/convex portions are parallel to each other, each of the centeraxes connecting a center of a base and a top point of each of theconcave/convex portions.
 13. The antireflection structure of claim 1,wherein each of the plurality of fine concave/convex portions has aheight equal to or larger than 0.4 times as large as a wavelength oflight of which reflection is suppressed.
 14. An optical devicecomprising the antireflection structure of claim
 1. 15. Anantireflection structure for suppressing reflection of light having awavelength equal to or larger than a predetermined wavelength, theantireflection structure comprising: a rough surface having a largersurface roughness than the predetermined wavelength and having anaperiodic roughness shape, wherein a plurality of fine concave/convexportions are further formed on the rough surface so as to be regularlyarranged within a smaller cycle than the predetermined wavelength,wherein a surface roughness of the rough surface in terms of a maximumheight roughness Rz defined by ISO4287:1997 is smaller than 100 μm, andwherein the rough surface is formed so that a distribution width ofcycles standardized with a center cycle of the roughness shape is equalto or larger than ⅔ times as large as the center cycle.